Ferroelectric Phase Transitions in Sn_<b>2</b>P_<b>2</b>S_<b>6</b>and Sn_<b>2</b>P_<b>2</b>Se_<b>6</b>Crystals

Abstract

Heat capacities of Sn 2 P 2 S 6 and Sn 2 P 2 Se 6 single crystals were measured at temperatures between 10 and 380 K for the former and between 10 and 350 K for the latter using an adiabatic calorimeter. The heat capacity anomaly due to the ferroelectric-paraelectric phase transition occurred at 337.25 ±0.10 K in Sn 2 P 2 S 6 . The anomalies corresponding to the ferroelectric-incommensurate and incommensurate-paraelectric phase transitions occurred at 193.24 ±0.10 and 220.07 ±0.15 K in Sn 2 P 2 Se 6 . The normal heat capacities for the compounds were estimated by least squares fitting of the Debye and Einstein functions to the experimental data supplemented by spectroscopic normal mode frequencies. The anomalous heat capacities gave the phase transition entropies of 8.6 JK -1 mol -1 for Sn 2 P 2 S 6 and 8.5 and 1.5 JK -1 mol -1 for Sn 2 P 2 Se 6 . The large transition entropies are consistent with an order-disorder mechanism for the ferroelectric-paraelectric phase transitions in Sn 2 P 2 S 6 and Sn 2 P 2 Se 6 .